The dyeing of polyester fibers is usually carried out by using a disperse dye in an acidic dye bath (having a pH of 4 to 6) at a temperature of 120-140.degree. C. The disperse dye is unstable in an alkaline dye bath, and its use in an alkaline dye bath makes it difficult to achieve a high reproducibility of dyeing with the same color-shade.
There has, however, arisen recently a new technical demand for the dyeing of polyester fibers in an alkaline bath having a pH of 8 to 10. It is based on the desire to obtain dyed products without having any seriously impaired reproducibility of dyeing even if any alkaline substance used for scouring or weight reduction prior to dyeing may be carried forward into a dye bath as a result of e.g. insufficient washing and make it alkaline, and the advantages of an alkaline dye bath, which dissolve any olygomer (a low-molecular component present in polyester fibers) remaining on cloth passed to the dyeing process, thereby preventing any trouble caused by coarse particles of any olygomer remaining after filtration during, for example, cheese dyeing, while the alkaline dye bath also makes it possible to prevent any coagulation of the dye, and remaining size, and thereby decrease the contamination of a can holding it.
There has also arisen recently a new technical demand for the one-bath dyeing of a mixture of polyester and cellulose fibers, particularly of polyester and rayon fibers, with a disperse dye and a metal complex type direct dye in an alkaline bath having a pH of 8 to 10. While it has also been usual to carry out the dyeing of a mixture of polyester and cellulose fibers in an acidic dye bath (having a pH of 4 to 6) at a temperature of 120-140.degree. C., the advantages of their dyeing in an alkaline bath include not only the advantages as stated above of the dyeing of polyester fibers in an alkaline bath, but also an improved degree of level dyeing, as the alkaline bath restrains the initial absorption of the direct dye, an improved feeling of the mixed polyester and rayon fibers as dyed, etc.
The use of an amino acid, or a derivative thereof has been proposed to prevent the decomposition of a disperse dye used for the absorption dyeing of polyester fibers, or fibers containing polyester fibers in an alkaline bath (Japanese Patent Publications Nos. Hei 7-53952 and Hei 7-18093). The use of an amino acid, or a derivative thereof, however, cannot be said to be satisfactorily effective for preventing the decomposition of a disperse dye, though it may certainly be effective to some extent or other. Moreover, the amino acids have the drawback of being difficult to use for the one-bath absorption dyeing of mixed polyester and cellulose fibers with a disperse dye and a metal complex type direct dye in an alkaline bath, since they hinder the exhaustion of the direct dye, or bring about a change of its hue.
Under these circumstances, there has been a desire to establish a dyeing method which enables the exhaustion dyeing of fibers containing polyester fibers with a disperse dye in an alkaline aqueous medium to be carried out with high reproducibility by preventing the decomposition of the disperse dye, and moreover, a dyeing method which enables the one-bath exhaustion dyeing of a mixture of polyester and cellulose fibers with a disperse dye and a metal complex type direct dye in an alkaline aqueous medium to be carried out with high reproducibility by preventing the decomposition of the disperse dye and without having any adverse effect on the direct dye.
We, the inventors of this invention, have made a great deal of study to solve the problems of the prior art as stated above, and arrived at this invention.